In concrete paving operations, after concrete is poured, it is commonly finished by drawing a tool, such as a screed head, over the surface of the concrete. The screed head smoothes the concrete surface before the concrete has cured completely. Similarly, after asphalt is laid, it is commonly leveled to a desired depth by drawing a tool, similar to a screed head, over the surface to finish the surface and control the thickness of the asphalt. Additionally, in grading operations, a surface is graded by drawing a blade of a grader over the surface. Although the physical configurations of the various types of screed heads and blades differ, the functions of these tools are analogous.
Typically, support structures including hydraulic cylinders support the ends of the tool on such machines. The hydraulic cylinders can be actuated to raise and lower the ends of the tool independently. It has been common to determine the elevation positions of the ends of the tool by using a laser transmitter and a pair of laser beam receivers that are mounted on masts at each end of the tool. The laser transmitter provides a rotating beam of laser light that is sensed by the receivers. The tool is raised and lowered under control of a system that sets the ends of the tool at vertical positions in dependence upon a desired elevation for the tool and the sensed elevation of the ends of the tool.
The laser transmitter may project a beam of laser light that rotates in a reference plane. The laser receivers detect the reference plane and the relative elevation of the ends of the tool with respect to the reference plane. A control system of the machine then actuates hydraulic valves to supply fluid to the hydraulic cylinders in response to these detected levels. As a result, the elevation of each end of the tool can be precisely controlled. Each of the receivers provides feedback to drive the hydraulics controlling the elevation of the end of the tool with which it is associated.
If one of the receivers is blocked by something at the worksite, such as, for example, a support column, the proper operation of the system is interrupted. This commonly occurs when concrete is being finished on the floor of a building interior. When a blockage occurs, there is a need to maintain the relative elevation of the ends of the tool as it is drawn toward the machine until the laser beam can be reacquired by both receivers. One approach to this problem is to set up two laser transmitters at the same elevation on opposite sides of the tool. In this way, if a column blocks one of the transmitters, the other transmitter is likely to be illuminating the receivers at the ends of the tool, thereby compensating for the blockage. Essentially, this eliminates all blind spots around the receivers. While generally effective, this prior art method is disadvantageous in that by requiring an additional transmitter, the cost of the equipment is increased. Further, this method increases significantly the time required to set up the equipment and eliminate the possibility of a column block with the second laser transmitter.
Other approaches have been used to deal with the problem of one of the receivers not receiving the reference beam of laser light. Such approaches may involve providing an additional sensor or additional sensors, the output from which are used when one of the laser receivers is blocked. For example, a gravity based cross slope sensor may be mounted on the tool to indicate the inclination of the tool between the laser receivers. In such a system, when the laser beam reception of one of the laser receivers is blocked, the system will be operated using the laser receiver output that is not blocked to control the vertical movement of the end of the tool with which it is associated. The vertical movement of the other end of the tool is controlled to maintain as constant the inclination of the tool sensed just prior to losing reception of the laser beam. When reception of the beam by both receivers is restored, normal operation of the system based on the two receivers is resumed.
A problem that results from the use of a gravity based inclinometer as an additional sensor input is that such sensors are subject to significant inaccuracies when the tool is subjected to lateral acceleration. In screeding operations, it is common for the operator of the machine to side shift the tool around columns as the tool is being pulled toward the machine. Since the sensitive axis of the gravity based cross slope sensor is parallel to the length of the tool, side shifting can cause noticeable acceleration along the sensitive axis of measurement. This dramatically affects the feedback of the cross slope sensor. In order to reduce the effects of noise and to compensate for some of the low frequency harmonics of machine vibration, the cross slope sensor output is typically supplied to a low pass filter. However, providing a low pass filter on the output of the gravity based cross slope sensor adds an inherent time lag to the system, however, that degrades the bandwidth performance of the blocked side. The low pass filter does not, however, limit the errors induced by lateral shifts in the tool, since these shifts result in fairly low frequency error signals. Another approach that is not subject to errors from lateral shifts is to provide extension sensors on the two hydraulic cylinders that control the vertical position of the two ends of the tool. When the beam to one of the laser receivers is blocked, the cylinder at that end of the tool is driven on the basis of the difference in cylinder extension sensor outputs, while the hydraulic cylinder at the other end of the tool is driven based on the received beam. While this avoids the problems associated with lateral shift acceleration, the accuracy achieved is not as great as desired.